The composition of exhaust gases produced by the combustion of hydrocarbon fuels is a complex mixture of oxide gases (NOx, SOx, CO2, CO, H2O), unburnt hydrocarbon gases, and oxygen. Measurement of the concentration of these individual constituents of exhaust gases in real time can result in improved combustion efficiency and lower emissions of polluting gases. In some cases, the concentration of one gas may influence or control the concentration of a second gas. In these situations, it may be required to know the concentration of the first gas in order to measure the concentration of a second, or even third, gas accurately. Various devices have been proposed to operate as exhaust gas sensors that have the capability of measuring the gas concentration of two or more gases in an exhaust stream.
One NOx sensor known in the art is configured as a flat plate multilayer ceramic package designed to include two or more chambers. The first chamber has electrodes attached to an oxygen ion-conducting electrolyte membrane to form an oxygen pump for removing oxygen from a flow of gas entering the sensor. The first chamber also catalyzes the decomposition of NO2 to NO and one-half O2. The oxygen pump in the first chamber also removes the oxygen formed by this process. Thus, in theory, the only oxygen-containing gas that enters the second chamber is NO. The second chamber includes a NO decomposing element that removes the oxygen from the NO using a second oxygen pump. The electrical current produced by the transport of oxygen from the decomposition of NO in the second chamber is correlated to the concentration of NO.
A number of concerns affect the commercial application of this known NOx sensor. For example, when the NOx concentration to be detected is low, residual oxygen can cause significant interference. In addition to the above, the signal current produced by the sensor is very small, thus making it susceptible to interference from the electronic noise commonly found in an automobile. Also, the flow of exhaust gas monitored by such sensors typically has pulsations in its flow rate caused at least in part by engine cylinder firings. This impairs the ability of the oxygen pump to effectively remove all of the free oxygen and may result in measurement error. This device may also contain a small diffusion aperture used to limit the passage of gas into the measurement chambers. This structure has been demonstrated to be prone to clogging during use.
Another known NOx sensor utilizes a similar flat plate multilayer ceramic package design. There are a few significant differences in the operation principle for this sensor; namely, the sensor is a mixed potential type rather than amperometric, and the first chamber is used to convert NO to NO2 and vice versa. It is well established that in mixed potential NOx sensors, the voltage signals generated from the gas species NO and NO2 are of opposite sign. As a result, it is difficult to distinguish a meaningful voltage signal when both gases are present since cancellation may occur.
Some sensor designs have attempted to address this problem by utilizing a flat plate multilayer package design with two separate chambers built into the sensor. Attempts have also been made to convert all of the NOx gas species into a single species with the use of an electrochemical oxygen pump that pumps oxygen into the first chamber to attempt to convert all of the gas to NO2. Other efforts conversely attempt to remove oxygen from the chamber and reduce all of the NO2 to NO. This “conditioned” gas then passes into the second chamber where the NOx concentration is measured by the voltage signal generated from a mixed potential type sensor.
There are a number of limitations to this approach that have hampered the commercialization of this configuration. One significant concern is the reproducibility of the conversion system to completely convert all the NOx gases into a single species under varying gas concentration conditions. In addition, the oxygen pump conversion cell tends to degrade with time, further contributing to the issue of reproducibility. Because the effects of these concerns are magnified in the low concentration range, this measurement approach is not well suited for detecting low concentrations of NOx gases.
Additional drawbacks common to both of the sensor mechanisms discussed above stem from the fundamental design of the flat plate ceramic multilayer system. Response times tend to be slow because of the complexity of the device requiring gas to first enter through a diffusion port, be conditioned in a first chamber, and then to diffuse into a second chamber. Achieving rapid gas exchange that can keep up with the dynamic environment of the engine exhaust is difficult in these configurations. Also, the corrosive nature of the gas itself and the fact that it bears fine particulates may result in the clogging of the diffusion controlling port, or at the very least, changes in the gas flow dynamics with time. Finally, pulsations in gas flow rates due to cylinder firings and the electrical noise typical of automobiles make it difficult to control and monitor the low voltage and current circuits associated with these devices.
Thus, it would be an improvement in the art to provide alternative configurations for NOx sensing elements usable in a NOx sensor system designed to address these and other considerations. Such a device is provided herein.